To obtain the soliton effect, several conditions must be satisfied simultaneously. Firstly, the pulses must be short: e.g., for 10 Gbit/s transmission, each pulse must have a duration at half-intensity of about 20 ps. The curve showing how the intensity I of each pulse varies as a function of time t must have a shape defined by the following expression: EQU I=4I.sub.o /(e.sup.t/.tau. +e.sup.-t/.tau.).sup.2
where I.sub.o is the peak intensity and .tau. is a duration that is proportional to the duration of the pulse at half-height. Secondly, the pulses must be "close to the Fourier limit", i.e. the product of the duration of a pulse multiplied by its spectral width must be less than a limit of about 0.7, and must be as close as possible to an ideal limit of 0.32. Finally, the intensity of the pulses must be maintained approximately constant during propagation. As far as the fiber is concerned, a suitable ratio must be established between firstly the product of a peak intensity of a light pulse multiplied by a constant representative of the Kerr effect, and secondly the product of the square of a spectrum width of the pulse multiplied by a constant representative of the chromatic dispersion effect appearing in each unit of length of the fiber. The latter constant is a chromatic dispersion per unit length, which, when multiplied by the length of the fiber, constitutes an overall chromatic dispersion therefor, i.e. a chromatic dispersion occurring between the input and the output of the fiber. To obtain the soliton effect, both the chromatic dispersion per unit length and the overall chromatic dispersion must be positive. This is achieved if the central wavelength of the pulse is greater than a wavelength which is characteristic of the fiber and which cancels the dispersion therein.
Using the soliton effect may offer considerable advantages in high-rate transoceanic links (6,000 km-9,000 km) that use binary encoding. Given that optical fibers present losses which cause natural attenuation of pulses, such long transmission distances may be achieved by propagating solitons along fibers whose losses are compensated by optical amplifiers. Such amplifiers are typically constituted by erbium-doped fibers. The amplifiers are distributed over the length of the link to limit the variations in the optical power of the pulses so that the pulses retain their soliton properties.
However, the noise inevitably emitted by the amplifiers limits the distances and the data-rates that are accessible because such noise gives rise to a degradation in the signal-to-noise ratio and to the appearance of "time jitter" in the pulses at the input of the reception member.
Such time jitter may be referred to as "Gordon-Haus" time jitter. It results from the fact that superposing amplification noise on a soliton is equivalent to modifying the central wavelength of the soliton. The change in wavelength causes a variation in the propagation speed along the fiber because of the chromatic dispersion thereof. The random nature of the noise causes a random variation in the speeds of the various solitons, and therefore a random time displacement of the pulses on reception. Such displacement, which constitutes said time jitter, gives rise to an increase in the transmission error rate.
The following two articles on this subject may be consulted:
Gordon and Mollenauer, "Effect of fiber nonlinearities and amplifier spacing on ultra-long distance transmission", J. Light. Technol., 9, 170 (1991); and PA1 Gordon and Haus, "Random walk of coherently amplified solitons in optical fiber transmission", Optics Lett., 11,665 (1986).
To minimize the error rate of a soliton transmission system, two opposing conditions must be simultaneously satisfied. A first condition is that the time jitter must be limited, and this would imply choosing a fiber that presents low chromatic dispersion per unit length. The second condition is that the signal-to-noise ratio must be maintained at a sufficiently high value. To achieve this, it is necessary for the pulses transmitted to have high energy. Such high energy Gives rise to large non-linear effects. Therefore, the second condition requires a line fiber to be chosen that has high chromatic dispersion per unit length, since such high chromatic dispersion per unit length is necessary for compensating the large non-linear effects, such compensation itself being necessary for obtaining the soliton effect.
Those two conditions imply choosing an optimum intermediate value for the chromatic dispersion per unit length so as to minimize the overall transmission error rate. When such an optimum value is achieved, it appears that the degradation of the signal-to-noise ratio and the time jitter both contribute considerably to obtaining the minimized overall rate.
That is why it has been proposed, in particular, to limit the time jitter. Two known methods have been proposed for that purpose. They both act at intermediate points along a very long link, i.e. distant from the transmission station and from the reception station.
A first method is disclosed in a document entitled "10 Gb/s soliton data transmission over one million kilometers", Nakazawa, Yamada, Kubota, Suziki, Elect. Letters, 27, 1270 (1991). In that method, the shape of each pulse is reconstituted from place to place so as to maintain both the soliton properties of the pulses and the time spacing between the pulses.
The second known method is described in a document entitled "Soliton Transmission Control" Mecozzi, Moores, Haus, Lai, Optics Letters, 16, 1841 (1992). It consists in performing frequency filtering on the pulses from place to place, to maintain a constant value for the central wavelength of each pulse.
Both those known methods are costly because they suffer from considerable technical difficulties: in the first method, the modulators must be synchronized, and, in the second method, the central wavelengths of the filters must be servo-controlled.